I watched The Emperor's New Groove for the first time in years the other day, and honestly it might be one of my favorite Disney films for just how different it is from the movies that came before it. The movie isn't as dramatic or heartfelt as the Disney Renaissance period, but that's why it works for me. It's a goofy, funny movie that has its own identity. It's just so different compared to what Disney was pumping out at the time, sometimes I forget it's from Disney at all.
And even beyond that, the movie just has a lot of charm. I actually like David Spade so Kuzco never came off as annoying to me, and I've always liked Kuzco's chemistry with Pacha. Yzma and Kronk are also funny as hell.
Would Kingdom of the Sun have been a better movie? Perhaps. But I don't know if it could have distinguished itself as well as The Emperor's New Groove did.